A relatively new generation of disk drive interface is the serial attached SCSI (SAS) architecture. The architecture is a point-to-point architecture using serial communication directly from the controller to a disk drive or through an expander switching matrix. Multiple devices can be connected at the same time. In contrast, conventional SCSI devices used a slower parallel communication using shared-access topologies in which only two devices are able to communicate at one time. However, SAS uses the conventional SCSI commands. Some additional benefits of the SAS architecture include convenient redundancy through the use of dual-ported disk drives, smaller form factor drives, thinner cables and smaller cable connectors.
One common configuration which addresses a need for high availability is illustrated in FIG. 1. A host device 102 is coupled to two storage controllers 104A, 104B, such as RAID controllers, which may also be known as initiators. The storage controllers 104A, 104B, in turn, are each coupled through SAS paths to both ports of a dual-port drive enclosure 110. Within the enclosure 110 are multiple disk drives 112 and two interface cards 116A, 116B, each of which includes an expander or switch module 118A, 118B. Each disk drive 112 is connected to one of the expanders 118A, 118B through one of two of the drive ports 114A, 114B. Connecting each controller 104A, 104B with each of the interface cards 116A, 116B permits maintenance of full operation of the drives 112 in the event that one of the storage controllers 104A, 104B or one of the interface cards 116A, 116B fails. An additional downstream enclosure 150 may be connected to the first enclosure 110 through additional dual SAS paths, with further downstream enclosures (not shown) connected in a trunk (daisy chain) topology. Alternatively, additional enclosures may each be connected to the first enclosure 110 in a star topology.
Despite the benefits of SAS technology, it remains desirable to further increase the density of disk drives relative to the volume of an enclosure while still maintaining full redundancy.